The following correction was printed in the Guardian's Corrections and clarifications column, Wednesday November 22 2006

The energy of all the particles in Cern's collider in Switzerland will "pack as much punch as a 400-tonne train travelling at 120mph", and not the energy in each proton, as we stated in error in the article below. The usual comparison for a single proton is with a mosquito landing on skin. The caption accompanying a photograph with the article in the newspaper wrongly stated that the number of expected collisions per second was 300m, rather than the correct figure of 800m given in the text.

At security posts dotted around the fields between the Jura mountains and Lake Geneva scientists are installing hi-tech retina scans above shafts descending 80m down - and leading to the largest scientific instrument ever built.

The machine is being bolted together inside a tunnel 17 miles (27km) long, and when the power is thrown on next year it will recreate conditions unknown for 14bn years since the extraordinary fireball that marked the beginning of the universe - the big bang which blasted time and space into existence.

In the coming months engineers using cranes will lower sections of detectors weighing several thousand tonnes into caverns carved within the tunnel. They will wire in some of the world's largest magnets and test run the machine's computer, built to handle a torrent of data equivalent to 150 times the content of the world wide web each year.

The machine, the Large Hadron Collider at Cern, Europe's particle physics laboratory, in Switzerland, was commissioned as a £4.2bn sledgehammer to crack some of the most compelling mysteries of the universe. Britain's stake in the project is enormous. This year alone £78m will be channelled into the experiment, where 600 British physicists are based.

The project may prise open extra dimensions and create baby black holes; it may reveal enigmatic "dark energy" that drives the expansion of the universe. It should certainly discover what some call the "God particle", finally answering the embarrassingly simple but elusive question of why things have mass.

In 1993 science minister William Waldegrave was so stumped by the notion of a God particle - correctly known as Higgs boson after the Edinburgh University physicist Peter Higgs who proposed it - that he offered a bottle of champagne to anyone who could explain it on one side of A4. The winning entry used the analogy of Margaret Thatcher gathering hangers-on as she moved through a cocktail party, to explain how Higgs bosons make other particles heavy by clinging to them like treacle. Finding the Higgs boson will confirm scientists' most complete theory of the universe and the matter from which it is created. "It's probably the closest to God that we'll get," said Jos Engelen, Cern's chief scientist.

Inside the collider vanishingly small protons, the particles at the heart of every atom, will be propelled to nearly the speed of light and slammed into other protons hurtling the other way. By the time they collide each proton will pack as much punch as a 400-tonne train travelling at 120mph. Every second an estimated 800m head-on collisions are expected, each unleashing a shower of subatomic debris for scientists to sift through.

Although the elusive Higgs particle may be created in collisions every day it will take enormous skill to spot them. Their existence is fleeting, each lasting less than a thousandth of a billionth of a billionth of a second.

Scientists will use the enormous 7,000-tonne Atlas detector, which sits inside a cavern large enough to house the nave of Westminster Abbey, to pick up other particles that can only be created when a Higgs boson vanishes from existence.

Other experiments will veer sharply into what has previously been the realm of science fiction. Some scientists believe the universe has more dimensions than the ones we know about. In one extra dimension gravity is believed to be exceptionally strong. If the collider momentarily wedges extra dimensions open, it could release a powerful tug of gravity that compresses matter so much it creates a miniature black hole.

Cern officials are keen to point out that there is no reason to be alarmed by artificial black holes. "You should not deduce that we are ready to build a black hole and Cern along with the planet will disappear, although this is a letter I receive every week," said Robert Aylmar, head of Cern.

Other detectors will investigate a theory called supersymmetry which predicts that there is a heavy invisible twin for every particle in the universe and which could explain why 90% of the material in the universe appears to be "missing" - a mystery that led scientists to name it "dark matter".

Valuable spin-offs from past research include the world wide web and the most advanced medical scanners found in hospitals. Professor Engelen admits there is no practical benefit in finding the Higgs Boson. "Even in my wildest imagination I can't think of this discovery having a practical application, but setting ourselves that goal, doing something so exceptionally difficult, has required us to be innovative technology-wise. I can very easily sell the idea of new and fundamental science using that argument, even though the Higgs itself is not going to let you make a better toothpaste," he said.